When Naomi Watts was a struggling actress, she never would have imagined that one day she would play Princess Diana, one of the most famous women in the world, even after her death.

In fact, the thought makes her laugh.

“Yeah, that would sound a bit silly wouldn't it,” said the actress at the New York premiere of the biopic “Diana” on Wednesday night.

Watts plays the Princess of Wales during roughly the last two years of her life. The story is based on the 2001 book “Diana: Her Last Love,” chronicling her relationships with heart surgeon Hasnat Khan and Dodi Fayed.

Cas Anvar, who plays Fayed, would often marvel on set about the way Watts embodied the essence of Princess Diana. In fact, he says she even stayed in character between takes.

Actress Naomi Watts attends the premiere of "Diana" hosted by The Cinema Society, Linda Wells and Allure Magazine at the SVA Theater on Wednesday, Oct. 30, 2013 in New York. (Evan Agostini/Invision/AP)

“It was quite surreal sometimes, but it was thrilling to be around, working with someone like that,” Anvar said. “She kept in character all the time, so I never actually got to experience the Naomi side of things,” he recalled. “I was more or less always interacting with Lady Di.”

Watts says she tried to stay in character not because she's “as disciplined as Daniel Day-Lewis,” but because the accent was so difficult to master.

Despite all her effort, few have been impressed with the film, which opens Friday. Reviews have been mostly negative thus far.

Naveen Andrews, who plays Dr. Khan, believes a big part of that is because Diana really was, as her nickname implies, the people's princess.

“Obviously in England, I think people feel a sense of ownership over her,” he said. “They did when she was alive. Now they do that she's passed. It's a testament to her power that she can generate so much emotion and feeling.”

Watts agrees: “Everyone feels they know her and they thought they had an opinion about who she was and their version of the story must be true and the comparisons that will be made inevitably.”

Anvar says he thinks the strong opinions over the film are a good thing.

“Personally I would rather be part of a project that inspires massive debate and controversy than a project that just fades away with a whimper, he said. “Any kind of uproar or upheaval usually is a good thing and indicative of a good story.”